Water is a chemical substance with a simple but stable molecular structure of H 2 O. It cannot be generated or eliminated under normal conditions, so the total mass of water in the whole world is constant. Water itself is transparent, tasteless, odorless, and nearly colorless, but can dissolve various inorganic and organic solutes, and carry fine particulate solids as it flows. All these foreign substances the water carries are called 'impurities' which can enter the water mass and conversely be removed through various natural or artificial actions. Water is essential to human and all living organisms, but any consumption and utilization of water cannot result in any true loss of the water mass. This means that after water use, the water molecules are still there and the only alteration is in their quality, location of existence, and existing state (liquid, gaseous or even solid forms). However, the usability of water may not merely depend on its quantity, but more importantly its quality. Contaminated water cannot be directly supplied for most purposes of water use. In this sense, the problem of water shortage we